Software nullification of transducer induced offset errors within a flow rate measurement system

Abstract

Fluid flow in a conduit is measured by placing a flow block in the conduit which introduces variations in the flowpath which cause local pressure variations. The pressure variations are converted to electrical signals by transducers, and the electrical signals are processed to obtain flow rate values. To provide a correction for transducer drift, the transducer signals are examined during a time when there is known to be zero flow in the conduit. The difference between the transducer signals during a time when there is zero flow is used as a measure of transducer drift. A value for the drift is stored in computer memory and used subsequently, when fluid flow is resumed. This invention is specifically intended for use in railroad airbrake systems, where it is used in a locomotive to provide an accurate and reliable measurement of air supplied to the brake pipe of the air brake system.

I claim: 
     
       1. A method of measuring the fluid flow rate in a fluid pressure communication conduit which, at certain known times, carries zero flow, said method comprising the following steps: (a) introducing local variations into a flow path of such fluid pressure communication conduit to produce a plurality of pressures at a plurality of locations, having a relationship indicative of such fluid flow rate;   (b) measuring said plurality of pressures at said plurality of locations using at least one pressure transducer to generate at least one signal indicative of said plurality of pressures;   (c) calculating such fluid flow rate using said at least one signal indicative of said plurality of pressures, and also using at least one signal indicative of a drift of said at least one pressure transducer; and   (d) in response to a signal indicating that such fluid flow has been stopped, using said at least one signal indicative of said plurality of pressures to obtain at least one signal indicative of said drift of said at least one pressure transducer and storing said at least one signal indicative of said drift for subsequent use in step (c) when such fluid flow is resumed.   
     
     
       2. The method of claim 1 further comprising the step of comparing a magnitude of said at least one signal indicative of said drift of said at least one pressure transducer with an offset error threshold signal to generate an offset error signal if said magnitude exceeds said offset error threshold signal. 
     
     
       3. The method of claim 1 wherein said plurality of pressures is two pressures. 
     
     
       4. The method of claim 3 wherein said at least one transducer is two transducers, producing two signals indicative of pressure. 
     
     
       5. The method of claim 3 wherein said at least one transducer is one differential transducer, producing one signal indicative of a difference between said two pressures. 
     
     
       6. The method of claim 3 wherein flow is calculated based on a square root of said difference between said two pressures. 
     
     
       7. The method of claim 1 wherein said local variations in said flow path are variations in an area transverse to flow in such fluid pressure communication conduit. 
     
     
       8. The method of claim 1 wherein such fluid is compressed air, and such fluid pressure communication conduit is a brake pipe supply line in a locomotive, said brake pipe supply line leading to a brake pipe supply valve of a railroad air brake system, and wherein said signal indicating that such fluid flow has been stopped is a charge cutout command given when said locomotive is no longer in service as a lead locomotive. 
     
     
       9. The method of claim 2 wherein such fluid is compressed air, and such fluid pressure communication conduit is a brake pipe supply line in a locomotive, said brake pipe supply line leading to a brake pipe supply valve of a railroad air brake system, and wherein said signal indicating that such fluid flow has been stopped is a charge cutout command given when said locomotive is no longer in service as a lead locomotive, and wherein said offset error signal is used to provide a warning of excessive transducer drift to the engineer of said locomotive. 
     
     
       10. A system for measuring the fluid flow rate in a fluid pressure communication conduit which, at certain known times, carries zero flow, said system comprising: (a) a portion of such fluid pressure communication conduit having local variations in a flow path to produce a plurality of pressures at a plurality of locations, having a relationship indicative of such fluid flow rate;   (b) at least one pressure transducer connected to receive as inputs said plurality of pressures obtained at said plurality of locations, and to generate at least one signal indicative of said plurality of pressures;   (c) means for generating a charge cutout signal which is activated when such fluid pressure communication conduit carries zero flow;   (d) means activated by said charge cutout signal to obtain at least one signal indicative of a drift of said at least one pressure transducer and for storing said at least one signal indicative of said drift of said at least one pressure transducer; and   (e) means for calculating such fluid flow rate based on said at least one signal indicative of said plurality of pressures and said at least one signal indicative of said drift of said at least one pressure transducer, and for generating a signal indicative of such fluid flow rate.   
     
     
       11. The system of claim 10 further comprising: (f) means for supplying at least one offset error threshold signal; and   (g) means for comparing a magnitude of said at least one signal indicative of said drift of said at least one pressure transducer with said at least one offset error threshold signal to generate an offset error signal.   
     
     
       12. The system of claim 10 wherein said plurality of pressures is two pressures. 
     
     
       13. The system of claim 12 wherein said at least one transducer is two transducers, producing two signals indicative of pressure. 
     
     
       14. The system of claim 12 wherein said at least one transducer is one differential transducer, producing one signal indicative of the difference between said two pressures. 
     
     
       15. The system of claim 12 further comprising means for calculating such fluid flow rate based on the square root of the difference between said two pressures. 
     
     
       16. The system of claim 10 wherein said local variations in said flow path are variations in an area transverse to the flow in such fluid pressure communication conduit. 
     
     
       17. The system of claim 10 wherein such fluid is compressed air, and such fluid pressure communication conduit is a brake pipe supply line leading to a brake pipe supply valve of a railroad air brake system. 
     
     
       18. The system of claim 10 wherein such fluid is compressed air, and such fluid pressure communication conduit is a brake pipe supply line in a locomotive, said brake pipe supply line leading to a brake pipe supply valve of a railroad air brake system, and wherein said charge cutout signal is activated when said locomotive is no longer in service as a lead locomotive. 
     
     
       19. The system of claim 11 wherein such fluid is compressed air, and such fluid pressure communication conduit is a brake pipe supply line in a locomotive, said brake pipe supply line leading to a brake pipe supply valve of a railroad air brake system, and wherein said charge cutout signal is activated when said locomotive is no longer in service as a lead locomotive, and wherein said offset error signal is used to provide a warning of excessive transducer drift to the engineer of said locomotive. 
     
     
       20. A system for measuring the fluid flow rate in a fluid pressure communication conduit which, at certain known times, carries zero flow, said system comprising: (a) a portion of such fluid pressure communication conduit having local variations in a flow path to produce a plurality of pressures at a plurality of locations, having a relationship indicative of such fluid flow rate;   (b) at least one pressure transducer connected to receive as inputs a plurality of pressures obtained at said plurality of locations, and to generate at least one signal indicative of said plurality of pressures;   (c) means for generating a charge cutout signal which is activated when such fluid pressure communication conduit carries zero flow;   (d) means activated by said charge cutout signal to obtain at least one signal indicative of a drift of said at least one pressure transducer and for storing said at least one signal indicative of said drift of said at least one pressure transducer; and   (e) a computer programmed to calculate such fluid flow rate based on said at least one signal indicative of said plurality of pressures and said at least one signal indicative of said drift of said at least one pressure transducer, and for generating a signal indicative of such fluid flow rate.